Radio detection and ranging or "radar", sound navigation and ranging or "sonar" and light detection and ranging or "lidar" (also known as "ladar") are important for many navigation, fuzing, law enforcement and collision avoidance applications. These are used for sensing the location and velocity of either a single object or a plurality of objects simultaneously, independent of conditions affecting detection. An increasingly important application of such sensing devices is in avoidance of collisions between, for example, land based vehicles such as automobiles, motorcycles, trucks and between sea based vehicles and further between airplanes, helicopters and other flying vehicles, including avoidance of fixed or moving objects. Lidar apparatus are also usefully employed for measuring speeds of vehicles because such apparatus provide narrower field of view than, for example, radar apparatus, allowing discrimination of individual vehicles within a group, for example.
Similar apparatus are employed for time domain reflectometry, seismic mapping, ultrasonic imaging for medical and other nondestructive testing and evaluation, as well as other pulse-echo or signal-echo apparatus.
Such sensing apparatus operate by generating and transmitting an interrogation signal and receiving an echo thereof. The delay between transmission and subsequent reception of a reflected component or echo of the transmitted signal is determined. This delay measures distances or ranges between the transmitter, the remote object and the receiver. Many characteristics (e.g., size, albedo, shape) of the remote object may be determined from detailed comparisons between transmitted and received signals.
An object which is being echo-ranged provides a large return signal if close to the echo-ranging transmitter and a small return when far away. To a first approximation the amplitude of the return echo signal varies inversely as a power of the distance between the transmitter/receiver and the target. Since the outgoing and returning pulses typically propagate with constant velocity, the return echo amplitude for targets at various distances also decreases approximately inversely as the square of time.
This relationship is illustrated by curve 52 in FIG. 1. The abscissa and ordinate of FIG. 1 are calibrated in arbitrary units, with times T.sub.0 and T.sub.1 corresponding to the initiation of and the duration of measurement interval T.sub.1, respectively, during which measurement occurs.
Times T.sub.B and T.sub.E mark the beginning and end, respectively, of a measurement epoch. A measurement epoch comprises an initial period T.sub.CAL and a subsequent measurement period T.sub.1. During initial period T.sub.CAL assessment of and calibration for measurement conditions may occur and the measurement pulse is launched. Other related events, such as receiver blanking (temporarily disabling the receiver to avoid receiver overloading by energy from measurement pulse transmission), also occur during this interval. During measurement period T.sub.1, the receiver is enabled and received signals are processed. Measurement period T.sub.1 concludes at time T.sub.E, when a subsequent measurement epoch may begin.
A commonly used technique in echo-ranging is dynamically adjusting the input attenuation (1/gain) and/or the detection threshold of the echo receiver to correspond to curve 52 of FIG. 1. In other words, if the object is close, the echo returns quickly and is large and the receiver attenuation or detection threshold should be high (gain can be low), and if the object is distant, the echo returns later and is weaker and the receiver attenuation or detection threshold should be low (gain must be high), as shown by curve 52. Ultimately, the received signal is extinguished when the reflecting object is sufficiently distant.
If the input attenuation (1/gain) and/or the detection threshold of the echo receiver is varied according to curve 52, one obtains a constant probability of target detection independent of distance. If the return signal lies above curve 52, it is detected, and if it lies below curve 52 it is not detected. Such a time varying detection threshold is referred to as an analog sensitivity time control (STC) curve or signal and is typically used for adjusting the receiver input sensitivity with time. Curve 52 represents an analog STC curve or signal and line 54 is a digital approximation of curve 52.
It is difficult in practice to accurately realize analog STC functions as exemplified by curve 52 by conventional analog techniques, such as R-C or R-L-C networks. Further, analog radio frequency techniques for providing such signals are inherently slow and become complex when it is necessary to accommodate arbitrary changes in the desired receiver gain or threshold by electronic methods. Often digital approximations such as 54 are made to arbitrary analog STC curves such as 52.
Examples of such analog radio frequency techniques for radar applications are discussed in U.S. Pat. No. 4,994,811, entitled "Sensitivity Time Control Device", by J. Moreira and in U.S. Pat. No. 4,415,897, entitled "Precision Control of RF Attenuators for STC Applications", by H. Kennedy, wherein use is made of digital-to-analog converters for controlling variable radio frequency attenuators. These radio frequency attenuators generate approximations to the desired STC curves in response to control signals.
Lidar apparatus does not require radio frequency signal processing because light pulses are transmitted and received. Signals resulting from detected light signals are processed at baseband frequencies. Conditions affecting propagation of transmitted and reflected light pulses complicate calibration of lidar apparatus.
FIG. 2 is a histogram comparing examples of expected lidar return signal levels from a target (hatched) and background signal levels from the ambient and/or noise (plain), both in arbitrary units, for three different situations. Return signal level 70 and background signal level 75 are representative of echo return signals under clear air conditions. Return signal level 80 and background signal level 85 are typical of what is observed under aerosol conditions, i.e., fine mists or thin clouds, while return signal level 90 and background signal level 95 represent what is often observed with bright background conditions, e.g., bright clouds.
The values represented by right hand (plain) histograms 75, 85 and 95 in the three data pairs of FIG. 2 show representative examples of the background signal levels typical for clear air, aerosol and bright background conditions, respectively. Ambient background signal levels for clear air 75 and aerosol 85 measurement conditions are approximately the same, while bright background ambient signal level 95 is typically much higher. For the size of particles normally found in naturally occurring clouds, backscattering of light is a relatively slowly changing function of the wavelength of the scattered light. For example, light having a wavelength of about 900 nanometers (e.g., light from a GaAs solid sate source) behaves very similarly to red light having a wavelength of 660 nanometers in the presence of clouds.
Comparison of left hand (hatched) histograms 70, 80 and 90 of the data pairs of FIG. 2 shows that the return signal levels are expected to be comparable for clear air 70 and bright background 90 scenarios, while return signal level 80 under aerosol conditions can be expected to be reduced appreciably. These different circumstances require different criteria for STC curves in order to optimally detect a target return signal in the presence of an ambient background signal.
An additional problem associated with the infrared light beams from prior art lidar apparatus employing solid state laser diodes has been that such light beams typically are not visible to the human eye without special tools and/or methods, making alignment and servicing of such apparatus difficult.
What are needed are improved means and methods for generating precise signals to realize STC curves for echo receivers which automatically account for varying ambient conditions over very short measurement intervals, corresponding to ranges of a few tens or hundreds of meters or less.